Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), and defects in near range vision usually associated with aging (presbyopia). The advantages of contact lens wear are numerous. Improved convenience and improved appearance in comparison to spectacle glasses are probably the two most important advantages to most consumers. However, contact lens wearing discomfort is one of the predominant reasons for discontinuation of contact lens wear. Contact lenses manufacturers have made a great effort in developing contact lenses with minimized wearing discomfort.
Contact lens wearing discomfort can be influenced by many factors. It is believed that the lubricity of a contact lens can be directly associated with contact lens wearing comfort, because there exist inevitably interactions between the lens's posterior surface and the corneal surface and between the lens's anterior surface and the posterior surface of the eyelid during the blink when being worn by a patient on eye. Lubricity describes the slipperiness of a surface, and generally can be characterized by its coefficient of friction (CoF) which is measured in vitro as the ratio of the horizontal friction force between two bodies and the force pressing them together (or normal force). The lower the CoF is, the more lubricious the surface. Recent studies indicate a correlation between the in vitro measurements of coefficient of friction (CoF) of hydrogel contact lenses and subjective comfort (Brennan NA., Optom Vis Sci 2009; 86:e-abstract 90957; Coles CML, Brennan NA., Optom Vis Sci 2012; 88:e-abstract 125603; Kern J, Rappon J, Bauman E, Vaughn B., Invest Ophthalmol Vis Sci 2013; ARVO E-Abstract 494; Jones L, Brennan N A, Gonzalez-Meijome J, Lally J, Maldonado-Codina C, Schmidt T A, Subbaraman L, Young G, Nichols J, members of the TIWoCLD, Invest Ophthalmol Vis Sci 2013; 54:TFOS37-70; Subbaraman L. N. and Jones L. W., Contact Lens Spectrum 28:28-33 (2013); Fonn D., Contact Lens Spectrum 28:28-33 (2013), herein incorporated by references in their entireties).
Unlike most physical properties of a material, CoF is not an intrinsic material property, but instead should be considered more correctly as a system property, because it depends upon many variables of a contact lens under testing and of a testing system, including materials used, a probing substrate against which a contact lens under test is moved, contact mode (e.g., a constant point of contact, a moving point of contact), normal force pressure, moving speed relative to each other, and lubricating fluid between the probing substrate and the testing lens, etc. Different methods has been developed/used in measuring in vitro the lubricity of contact lenses, such as, a tribometer (Rennie A. C., Dickrell P. L., Sawyer W. G., Tribology Letters 2005, 18:499-504; Roba M., Duncan E. G., Hill G. A., Spencer N. D., Tosatti S. G. P., Tribology Letters 2011, 44:387-97; U.S. Pat. No. 6,940,580, herein incorporated by references in their entireties), atomic force microscopy (Kim S. H., Marmo C., Somorjai G. A., Biomaterials 2001, 22:3285-94; Kim S. H., Opdahl A., Marmo C., Somorjai G. A., Biomaterials 2002, 23:1657-66, herein incorporated by references in their entireties), an inclined plane method (U.S. Pat. No. 8,480,227, herein incorporated by reference in its entirety), lubricity ratings based on digital rubbing of lenses between the fingers (U.S. Pat. No. 8,480,227). However, those previously reported methods are not suitable for high throughput in vitro measurements of friction of coefficient and for generating lubricity date. The development and production of silicone hydrogel contact lenses with a highly lubricious surface could benefit greatly from a predictive method for selection a lens formulation for forming silicone hydrogel contact lenses having a target lubricity of a coating material and formation of a coating on a contact lens without undue reliance on clinical trials. While one cannot eliminate such clinical trials, better predictive models will streamline selection processes of lens forming materials or coating materials by reducing the number of different options to be tested. They would also provide a means for distinguishing useful and beneficial lens forming materials and curing conditions from lens forming materials and curing conditions that are not so useful or beneficial for obtaining silicone hydrogel contact lenses with a highly lubricious coating during development and production of contact lenses, or useful and beneficial coating materials and coating conditions from coating materials and coating conditions that are not so useful or beneficial for obtaining a highly lubricious coating during development and production of contact lenses.
Therefore, there is still a need for methods for determining in vitro CoF of a contact lens and for developing and producing contact lenses with a target lubricity, involving use of a lubricity test of the invention.